The composite video signal for a color television program includes luminance and chrominance information which occurs during the image interval of a horizontal scan line. This information is decoded by appropriate receiver luminance and chrominance processing circuitry and is used to control operation of respective color designated electron beam producing structures. The composite video signal also contains information in a video blanking interval of each horizontal scan line. The blanking interval includes horizontal sync pulses and a "back porch" region which incorporates a chrominance burst signal comprising a number of cycles at the color subcarrier frequency, which is approximately 3.58 MHz in the U.S. This chrominance burst signal is used to synchronize the 3.58 MHz color oscillator of the receiver so that the receiver-decoded chrominance information accurately reflects the original program information. The burst signal is applied to the appropriate chrominance processing circuitry via a burst gate which desirably passes only burst information. The burst gate is energized by a burst gate keying pulse which occurs during the back porch interval of the composite video signal.
The composite video signal is also utilized by luminance signal processing circuits to provide the proper drive level of the electron beam producing structures. The burst signal is removed from the composite video signal prior to the application of the video signal to the luminance processing circuits. The signal level of the composite signal during the back porch portion of the blanking interval is designated as a reference blanking level. This signal is used in some applications, to determine a black level reference to control the brightness of the reproduced picture. U.S. Pat. No. 4,197,557, issued to Tuma et al. and entitled "Brightness Control Circuit Employing a Closed Control Loop" describes an arrangement which compares the back porch level with a reference voltage determined by the receiver brightness control. The back porch level is then clamped to bring it into a desired relationship with the brightness reference voltage. Clamping of the blanking reference (back porch) level in this manner prevents video signal blanking level variations from causing undesirable changes in the picture brightness.
The aforementioned luminance-channel brightness control circuit and previously described chrominance-channel color burst gate utilize information contained in the back porch region of the horizontal video blanking interval. The back porch signal is sampled by the appropriate luminance and chrominance processing circuitry, which is energized by a keying pulse which occurs during the back porch interval. In some applications such as with the use of an integrated circuit, one keying pulse is used to energize both the brightness control or back porch clamp circuit and the burst gate. The timing and duration of this keying pulse is very important. The pulse must be of sufficient duration to key enough back porch information into the luminance and chrominance processing circuits. Also, the leading edge of the keying pulse must not occur too soon or the brightness control circuits may respond to the horizontal synchronizing pulse, rather than to the back porch information. The trailing edge of the keying pulse must not occur too late; otherwise, video information may be keyed into the burst gate, disrupting its operation.
U.S. Pat. No. 4,051,518, issued to Sendelweck and entitled "Burst Gate Pulse Generator" describes a keying pulse generator which produces an output pulse in timed relationship to horizontal synchronizing pulses. Since the horizontal synchronizing pulses occur at accurately timed fixed intervals, the resultant keying pulses will also be accurately timed. During some receiver operating conditions, however, such as when a weak signal is received or the weather is unfavorable, a significant amount of noise may be present in the video signal. This noise may obscure or even obliterate the horizontal sync pulses. In these situations, a horizontal sync responsive keying pulse generator may generate an improperly timed pulse or may generate no pulse at all. Such a situation may result in severely degraded receiver performance.
Additionally, if the keying pulse is also used by the previously described back porch clamp or brightness control circuit, the absence of a pulse may cause undesirable brightness variations or gray scale discoloration. It is important therefore that a keying pulse be provided under all receiver operating conditions. It is possible to generate a pulse triggered by horizontal retrace pulses, which are always present. However, due to deflection circuit loading variations, the retrace pulses may shift position from line to line. This may cause inaccurate timing of the keying pulses under some conditions.